A vulcanized rubber comprising a siliceous pigment, a rubber and an organic coupling agent having an active olefinic linkage

ABSTRACT

The novel rubber compound and tire tread made therefrom include a silica-filled unsaturated rubber compound and a reactive olefinic coupling agent such as 5-triethoxysilyl norbornene. The coupling agent is a compound which forms a bond or connection between the rubber polymer and the silica.

United States Patent Doran et al.

Wagner, Barberton; Henry C. Stevens, Akron, all of Ohio PPG Industries, Inc., Pittsburgh, Pa.

[73] Assignee:

[22] Filed: July 7, 1969 [21] Appl. No.: 839,644

[52] US. Cl ..l52/330, 23/182, 106/288 B,

[51 Int. Cl. ..B60c 5/00, C08d 13/20 [58] Field of Search 106/288 B, 309; 23/182;

[56] References Cited UNITED STATES PATENTS 3,350,345 10/1967 Vanderbilt et al. .....260/4l.5

[ 51 May 23, 1972 Kraus, Reinforcement of Elastomers (lnterscience) (N.Y.) (196 p. 417. TS 1925 K7.

Primary Examiner-Morris Liebman Assistant Examiner-H. H. Fletcher An0meyChisholm & Spencer ABSTRACT The novel rubber compound and tire tread made therefrom include a silica-filled unsaturated rubber compound and a reactive olefinic coupling agent such as S-triethoxysilyl norbomene. The coupling agent is a compound which forms a bond or connection between the rubber polymer and the silica.

14 Claims, No Drawings VULCANIZED RUBBER COMPRISING A SILICEOUS PIGMENT, A RUBBER AN ORGANIC COUPLING AGENT HAVING AN ACTIVE OLEFINIC LINKAGE This invention' relates to unsaturated rubber compounds and tire treads and, more specifically, this invention relates to silica-reinforced unsaturated rubber compounds and tire treads.

Previously known silica-reinforced rubber compounds prov vided certain highly desirable characteristics; for example, silica-reinforced rubber tires are known to have good resistance to tearing, cutting, flex cracking, aging and skidding. In addition, silica imparts a light color to the tires. Thus, white or colored tires may be made from silica-reinforced vulcanizates.

over previous silica-reinforced rubber compounds; for exam- 1 ple, improved cure time, viscosity, modulus values, tensile strength, compression set, and hysteresis or heat build-up.

The rubber compound of the present invention includes a finely divided reinforcing siliceous pigment, a coupling agent,

and a rubber polymer. Furthermore, the rubber compound may contain any of the various additives conventionally included in rubber compounds, such as typically used in making tire treads, for example, accelerators (e.g., guanidines), activators (e.g., zinc oxide), oil extenders (e.g., a mixture of 76 percent aromatic hydrocarbons and 26 percent naphthenic hydrocarbons manufactured and sold by Sun Oil Company under the trademark Sundex 5 3), curatives (e.g., sulfur), etc.

The present invention principally contemplates rubber compounds for mechanical rubber goods such as belting, motor mounts and tires. The rubber compound of the present invention is especially desirable for use in tire treads for pneumatic tires and treads for other types of tires. The present invention principally contemplates the type of unsaturated rubber polymers or elastomers typically used in tire treads; for example, styrene butadiene rubber, butadiene rubber, and natural rubber. However, the broader aspects of the present invention are applicable to other types of unsaturated rubber such as EPDM rubber. The rubber polymers used in the present invention preferably should have at least 4 mole percent unsaturation and may have in excess of 70 mole percent unsaturation and may be as high as 100 mole percent unsaturation. Typical styrene butadiene rubber has about 75 to 80 mole percent unsaturation.

The siliceous pigments preferably employed in this invention are precipitated, hydrated silica and are obtained by the precipitation of a soluble silicate, for example, silica produced according to the method described in US. Pat. No. 2,940,830. These pigments have an SiO content of at least 50 percent and usually greater than 80 percent by weight on an anhydrous basis. The siliceous pigment should have an ultimate particle size in a range of 50 to 10,000 angstroms, preferably between 50 and 400 and, more preferably, between 150 and 300 angstroms. The BET surface area of the pigment as measured using nitrogen gas is preferably in the range of 50 to 600, usually 70 to 300 square meters per gram. The BET method of measuring surface area is described in the Journal of the American Chemical Society, Vol. 60, page 304 (1930). A

typical commercial form of such reinforcing siliceous pigment is manufactured and sold by PPG Industries, Inc., under the trademark Hi-Sil 233. b

According to the present invention, it has been found that excellent rubber compounds and especially rubber tire treads can be prepared by the addition of a coupling agent to the rubber composition.

A coupling agent is a material that forms a connection or bond between, in this case, silica and rubber. The present invention is directed to the use of an active olefinic compound such as S-triethoxysilylnorbomene, isoprene-triethoxysilane adduct, 4(triethoxysilyl ethyl)cyclohexene, 6-triethoxysilyl-2- ethylidene norbornane, or 5-triethoxysilyl-2-ethylidene norbomane as the coupling agent.

The coupling agent used in the present invention is a difunctional compound which may be represented by a general formula of the type A-B. A represents a functional group that is capable of chemically or physically bonding to a silica particle, such bond preferably being between a silicon atom of the coupling agent and an oxygen atom of the silica particle. In the broader aspects of the present invention, functional group A may provide a bond through any member of group IV in the periodic table, particularly carbon, silicon, germanium and tin. Alternatively, the bond may be through a member of group V in the periodic table, especially nitrogen or phosphorous. Furthermore, the bond may be through any multivalent element in rows 2 and 3 in the periodic table, especially oxygen and sulfur. B represents a functional group that is capable of chemically or physically bonding to the rubber polymer, such bond being between a carbon atom and the rubber polymer.

Group A is a group that will provide a multivalent atom which is bonded to group B and which will bond with the silica particle. For example, the coupling agent A-B may include a group A as follows:

The letter designations will have the same meaning whenever used herein. D is any member selected from groups IV or V in the periodic table, especially carbon, silicon, germanium, tin, nitrogen or phosphorous, or any multivalent element in rows 2 or 3 in the periodic table, especially oxygen and sulfur; X is any moiety that will react with the silanols present in the silica, thereby permitting the multivalent atom of the coupling agent to bond to an oxygen atom of the silica. X can be, for example, a halide. Alternatively, X can be an oxy-group OR. R is H, alkyl, cycloalkyl, aryl, aralkyl, alkaryl, or acyl group, or a member of these groups bearing substituents such as one of the halides. R is preferably a hydrocarbon group containing between 1 and 10 carbon atoms. The letter d is equal to the valence of D.

The preferred group A is a silane group or any group that will provide a silicon atom that bonds with the silica particle. The coupling agent A-B may include a group A as follows:

The letter n is 1, 2 or 3. The functional group A is preferably 'trimethoxysilyl -Si(OCH;,) or triethoxysilyl -Si(OCH CH Furthermore, the functional group A could be as follows:

used herein with respect to the coupling agent, the term active olefinic linkage means an olefin having an internal double bond or, in other words, the carbon atoms of the double bond are other than terminal carbons. The active olefinic linkage is an olefinic linkage that is more reactive than that of ethylene. The reactivity of the olefinic linkage should compete favorably with the reactivity of the olefinic linkage in the rubber polymer. The more active the olefinic linkage of the coupling agent, the more effective is the coupling agent. Several factors affect the activity of the olefinic linkage of the coupling agent. For example, with open-chain olefins, the amount of substitution on the carbon atoms of the double bond is of importance. Preferably, such carbon atoms each have at least one methyl group attached thereto. Although the hydrogen atoms themselves may not enter into the reaction, yet the greater the number of allylic hydrogens, the more active will be the olefinic linkage. Certain cyclic olefms provide more active olefinic linkages than do open-chain olefins. The olefinic linkage is especially active if the double bond suffers angle strain such as in a norbornene or cyclobutene. This degree of strain can be measured as in the work of R. B. Turner, W. R. Meador and R. E. Winkler, Journal of the American Chemical Society, Vol. 79, page 41 l6 (1957). In any event, the activity of the olefinic linkage of the coupling agent must be sufficiently great to result in a bonding between the coupling agent and the rubber polymer upon vulcanization. When the coupling agent is reacted with the rubber polymer, one bond of the double bond breaks such that the carbon atom bonds directly with the rubber polymer. The following groups are typical of those that would provide such a reactive olefinic linkage:

present invention In the broader aspects of the present invention, group A Group B is an olefin including an active olefinic linkage. As

Preferred examples of the coupling agent of the are:

L II IH (OHaOhSisnon o);

and

(CH30)3Si Unless otherwise indicated, the hereinafter-described ingredients will be stated as parts by weight. Furthermore, the formulations will be based on parts of rubber. Typically, the rubber composition used in the tire treads of the present invention may include 100 parts of rubber, such as a styrene butadiene rubber; 5 to 100 parts of silica (preferably 40 to 90 parts), such as Hi-Sil 233 (trademark of PPG Industries, Inc.) reinforcing siliceous pigment; 0.1 to 15 parts coupling agent (preferably 0.3 to 3 parts), such as S-triethoxysilylnorbornene; and conventional sulfur or peroxide curatives. The sulfur curative may include 0.5 to 3 parts sulfur, 2 to 5 parts zinc oxide, and 0.5 to 2 parts accelerator. A peroxide curative may .include 1 to 4 parts dicumyl peroxide, such as Dicup 40C (trademark of Hercules Powder Company, Inc.). Although not essential, other conventional rubber additives are not detrimental. Other conventional rubber additives would include carbon blacks, oils, plasticizers, antioxidants, and colors.

The coupling agent can be added in any of several stages of rubber compounding. For example, satisfactory rubber compositions can be obtained when the difunctional organic compound, preferably a trialkoxysilyl olefin, is reacted with the silica prior to incorporating the silica into the rubber batch such as by coating the silica with the coupling agent. Satisfactory rubber compositions can also be obtained when the coupling agent is incorporated into a co-precipitated silica latex masterbatch. The coupling agent can be reacted with the rubber prior to incorporation of the silica therein or it can be added to the rubber batch together with silica and various other additives during the Banbury mixing. The coupling agent may be in any one of several forms when it is reacted with the rubber or silica; for example, it may be in the vapor state. Alternatively, it can be added in a substantially pure state, dissolved in an organic solvent, or dissolved or suspended in water.

Preferably, the coupling agent is permitted to react with the silica and the rubber prior to the incorporation of any of the polar additives, particularly soaps, metal oxides (especially zinc oxide), amines, glycols, and accelerators (especially guanidine) into the rubber batch. The following examples are illustrative of the present invention.

EXAMPLE 1 Rubber Compounds l-A and l-B were prepared according to the following formulations. Rubber Compound l-A is a preferred example of the present invention. The ingredients shown in Table l were mixed in a Banbury for about 5 minutes at a temperature of between 300 F. and 320 F. Rubber Compound l-A typically illustrates a rubber compound used in the tire treads of the present invention. Rubber Compound [-3 is a control compound and did not contain a coupling agent. Carbon black (8-300 Black) was added to both compounds for the purpose of coloring.

Standard ASTM designation. Approximately 75 mole percent unsaturation.

*l'rademark, PPG Industries, Inc. A precipitated, hydrated silica having an ultimate particle size of 200 angstroms, a BET surface area of approximately I50 square meters per gram, and containing SiO,87.5%, Ciao-0.75%, R,O -0.95%,

Nam-1.6%, a weight loss at C. of 6.3%, and the balance is bound water.

Trademark, Sun Oil Company. An aromatic oil. "Trademark, United States Rubber Company. A physical mixture containing 65% ofa complex diarylamine ketone reaction product and 35% ofa commercial N,N'-,

diphenyl-p-phenylene diamine. Standard ASTM designation.

TABLE II l-A l-B Z.2'-Benzothiazole disulfide 1.5 1.5 Di-ortho-tolylguanidine 1.5 1.5 Sulfur 2.75 2.75 ZnO Master (containing H3 6 6 SBR 1502 and 2/3 zinc oxide) The resulting compounds had the following characteristics:

5 Tested by Three-T Fleet. Inc., substantially as described in Patent N 0. 3,397,583. The results are accurate to within 5 points.

In determining the Road Wear Index, a tread band or portion was prepared of the Rubber Compounds I-A and 'I-B. The tread bands were applied to a new tire carcass. The tire cure included preheating the rubber compounds for 35 minutes at 210 F., then placing same in the mold for 17' minutes at 331 F. The tire was mounted on an automobile which was driven at 70 miles per hour on a hard-surfaced road for 4,400 miles. The road traveled was flat and included a combination of curved portions and straightportipns. tes t is designated as being a combination of slow and fast wear. The amount of tread lost during the test is determined and the Road Wear Index shows a comparison of the wear of each of the tread bands using the control compound as the standard of reference. The tire size was 7.35 by 14 inches. The load was 1,160 pounds per square inch on each tire. The tires were inflated to 32 pounds per square inch gauge.

The following table shows further characteristics of Rubber; Compounds l-A and I-B. f

TABLE IV (Compounds Cured for 30 Minutes at 300 F.)

'Tested according to ASTM D-412. "Tested according to ASTM D-3 l4.

Tables Ill and IV show that the Rubber Compound l-A has substantially improved characteristics for tire tread. The improved characteristics include cure time, viscosity, tensile strength, modulus values, compression set, Pico Abrasion index, and Road Wear Index. The improved Road Wear Index is of particular importance. Examination of the Road Wear Index values shows that the Rubber Compound lA. which is one preferred embodiment of the present invention, has a resistance to road wear about 20 percent greater than the Road Wear Index of Compound l-B which is a silica-reinforced rubber compound without the coupling agent.

EXAMPLE II The following example illustrates the effectiveness of several different coupling agents in rubber compounds in which the rubber polymer is a very highly unsaturated rubber polymer, namely, a styrene-butadiene rubber. The rubber compounds were prepared by mixing 100 parts SBR 1,502 and 60 parts Hi-Sil 233 (trademark of PPG Industries, Inc.) in a Banbury for about 5 minutes at a temperature of between 300 F. and 320 F. The rubber batch was further mixed on an open mill for about 10 minutes at 180 F. During mixing on the mill, the following ingredients were added: 1 part phenyl-betanaphthylamine; 4 parts zinc oxide; 0.75 parts benzothiazole disulfide; 1.25 parts di-ortho-tolylguanidine and 2.50 parts sulfur. Compound Il-A was a control compound and did not contain a coupling agent. The Rubber Compounds "-13 through lI-E each further included a coupling agent which was added during Banbury mixing. The coupling agents used were as follows: Compound II-B contained 1.57 parts of S-triethoxysilylnorbomene Compound II-D contained 1.66 parts 4-(triethoxysilyl ethyl) Compound lI-E contained 1.74 parts triethoxysilyl2-ethylidene norbornane =CHCH 2 5 3) a- The resulting compounds had the following characteristics when tested according to the standard ASTM procedures.

Pico 300% 100% Abrasion Tensile Modulus Modulus Compound Index psi psi psi ll-A (Control) 75 3230 805 330 11-15 8 8 3510 1 1 10 360 ll-C 83 3630 1050 350 ll-D 3570 980 335 11-15 84 3550 1020 350 substantial improvement in abrasion resistance, tensile.

strength and modulus values over previous silica-filled styrene butadiene rubber. For example, Compound ll-B shows an improvement over the control Compound lI-A of more than 17 percent in the Pico Abrasion index and nearly 40 percent in 300 percent modulus.

EXAMPLE Ill The following example illustrates the effectiveness of various coupling agents in an ethylene propylene rubber polymer. The ethylene propylene rubber polymer used was EPDM*(*ASTM designation.) Epsyn-40-A**(**Trademark of CoPolymer Rubber and Chemical Corporation.) which has an unsaturation of about 4 mole percent. The rubber compounds were each prepared by mixing 100 parts of rubber polymer with 50 parts of Hi-Sil 233 (trademark of PFC Industries, Inc.) on a Banbury at 310 F. for 5 minutes. The rubber batch was further mixed on an open mill for about minutes at 180 F. During mixing on the mill, the following ingredients were added: 1 part phenyl-beta-naphthylamine; 5 parts zinc oxide; l.0 part stearic acid; 1.75 parts zinc dibutyl dithiocarbamate; 0.45 part tetraethylthiuram disulfide; 2.0 parts sulfur and 0.5 part benzothiazole disulfide. Since Compound llI-A was a control compound, it did not contain a coupling agent. The Rubber Compounds Ill-B through III-D each further included a coupling agent which was added during Banbury mixing. The coupling agents used were as follows: Compound Ill-B contained 1.54 parts of S-triethoxysilyl norbomene. Compound Ill-C contained 1.67 parts of 4-(triethoxysilyl ethyl) cyclohexene. Compound Ill-D contained 1.74 parts of triethoxysilyl-Zethylidene norbomane. The resulting compounds had the following characteristics when tested according to the aforedescribed ASTM procedures.

The above table illustrates that ethylene propylene rubber compounds containing the coupling agent of the present invention have improved properties of abrasion resistance, tensile strength, and modulus. For example, the Rubber Compoun'd Ill-D, containing triethoxysilyl-2ethylidene norbornane had a Pico Abrasion index which was more than 50 percent greater tlian that of the control Compound Ill-A. Although the present invention has been described with reference to the specific details of particular embodiments thereof, it is not intended thereby to limit the scope of the invention, except insofar as the specific details are recited in the appended claims.

We claim:

1. A vulcanized rubber compound including the reaction product of an unsaturated rubber polymer; a finely divided, reinforcing, siliceous pigment; and from about 0.1 to parts of an organic coupling agent per 100 parts of rubber polymer, said organic coupling agent having the general formula A-B, wherein A represents a functional group capable of reacting to bond to the siliceous pigment, and B represents a functional group capable of reacting to bond to the rubber polymer, said functional group B being an active olefinic linkage having an internal double bond 7' V 2. The compound as defined in claim 1 wherein A provides a bond with the siliceous pigment through an atom selected from the group consisting of carbon, silicon, germanium, tin, nitrogen, phosphorous, oxygen and sulfur.

3. The compound as defined in claim 1 wherein the coupling agent is a trialkoxysilyl olefin.

4. A vulcanized rubber compound including the reaction product of an unsaturated rubber polymer; a finely divided, reinforcing, siliceous pigment; and an organic coupling agent having the general formula A-B, wherein A represents a functional group capable of reacting to bond to the siliceous pigment, and B represents a functional group capable of reacting to bond to the rubber polymer, said functional group B being an active olefinic linkage having an internal double bond, and wherein the rubber compound includes by weight 100 parts of rubber polymer, from 5 to 100 parts siliceous pigment, and from 0.1 to 15 parts coupling agent.

5. A vulcanized rubber compound including the reaction product of an unsaturated rubber polymer; a finely divided, reinforcing, siliceous pigment; and an organic coupling agent having the general formula A-B, wherein A represents a functional group capable of reacting to bond to the siliceous pigment, and B represents a functional group capable of reacting to bond to the rubber polymer, said functional group B being an active olefinic linkage having an internal double bond, and wherein the rubber compound includes by weight 100 parts of unsaturated rubber polymer, from 40 to parts siliceous pigment, and from 0.3 to 3 parts coupling agent.

6. A vulcanized rubber compound including the reaction product of an unsaturated rubber polymer; a finely divided, reinforcing, siliceous pigment; and from about 0.1 to 15 parts of an organic coupling agent per parts of rubber polymer,

said coupling agent being selected from the group consisting 8. cuaonsiw 7. The rubber compound as defined in claim 6 wherein said unsaturated rubber polymer is a member selected from the group consisting of EPDM rubber, styrene-butadiene rubber, butadiene rubber, and natural rubber.

8. The rubber compound as defined in claim 6 wherein said unsaturated rubber polymer has at least 4 mole percent unsaturation.

9. The compound as defined in claim 4 wherein the rubber polymer is EPDM and the connecting agent is S-triethoxysilylnorbomene.

10. A tire tread comprising a vulcanized rubber compound including the reaction product of an unsaturated rubberpolymer; a reinforcing amount of a finely divided, reinforcing,

siliceous pigment; and from about 0. l to parts of an organiccoupling agent, said coupling agent per 100 parts of rubber polymer having the general formula A-B, wherein A represents a functional group capable of reacting to bond to the siliceous pigment, and B represents a functional group capable of reacting to bond to the siliceous pigment, and B represents a functional group capable of reacting to bond to the rubber polymer, said functional group B being an active olefinic linkage having an internal double bond.

11. The tire tread as defined in claim 10 wherein said unsaturated rubber polymer is a member selected from the group consisting of styrene-butadiene rubber, butadiene rubber, and natural rubber.

12. A tire tread comprising a vulcanized rubber compound including the reaction product of an unsaturated rubber polymer; a finely divided, reinforcing, siliceous pigment; and

an organic coupling agent having the general formula A-B,

wherein A represents a functional group capable of reacting to bond to the siliceous pigment, and B represents a functional group capable of reacting to bond to the rubber polymer, said functional group B being an active olefinic linkage having an internal double bond, wherein said rubber compound includes 40 to parts siliceous pigments, 0.3 to 3 parts coupling agent, and parts rubber polymer.

13. The tire tread as defined in claim 12 wherein said coupling agent is a member selected from the group consisting of S-triethoxysilyl norbornene, isoprene-triethoxysilane adduct, 4-( triethoxysilyl ethyl)cyclohexene, 6-triethoxysilyl-2- ethylidene norbornane, and S-triethoxysilyl-2-ethylidene norbornane.

14. The tire tread as defined in claim 11 wherein the rubber polymer is EPDM and the connecting agent is S-triethoxysilylnorbomene. 

2. The compound as defined in claim 1 wherein A provides a bond with the siliceous pigment through an atom selected from the group consisting of carbon, silicon, germanium, tin, nitrogen, phosphorous, oxygen and sulfur.
 3. The compound as defined in claim 1 wherein the coupling agent is a trialkoxysilyl olefin.
 4. A vulcanized rubber compound including the reaction product of an unsaturated rubber polymer; a finely divided, reinforcing, siliceous pigment; and an organic coupling agent having the general formula A-B, wherein A represents a functional group capable of reacting to bond to the siliceous pigment, and B represents a functional group capable of reacting to bond to the rubber polymer, said functional group B being an active olefinic linkage having an internal double bond, and wherein the rubber compound includes by weight 100 parts of rubber polymer, from 5 to 100 parts siliceous pigment, and from 0.1 to 15 parts coupling agent.
 5. A vulcanized rubber compound including the reaction product of an unsaturated rubber polymer; a finely divided, reinforcing, siliceous pigment; and an organic coupling agent having the general formula A-B, wherein A represents a functional group capable of reacting to bond to the siliceous pigment, and B represents a functional group capable of reacting to bond to the rubber polymer, said functional group B being an active olefinic linkage having an internal double bond, and wherein the rubber compound includes by weight 100 parts of unsaturated rubber polymer, from 40 to 90 parts siliceous pigment, and from 0.3 to 3 parts coupling agent.
 6. A vulcanized rubber compound including the reaction product of an unsaturated rubber polymer; a finely divided, reinforcing, siliceous pigment; and from about 0.1 to 15 parts of an organic coupling agent per 100 parts of rubber polymer, said coupling agent being selected from the group consisting of
 7. The rubber compound as defined in claim 6 wherein said unsaturated rubber polymer is a member selected from the group consisting of EPDM rubber, styrene-butadiene rubber, butadiene rubber, and natural rubber.
 8. The rubber compound as defined in claim 6 wherein said unsaturated rubber polymer has at least 4 mole percent unsaturation.
 9. The compound as defined in claim 4 wherein the rubber polymer is EPDM and the connecting agent is 5-triethoxysilylnorbornene.
 10. A tire tread comprising a vulcanized rubber compound including the reaction product of an unsaturated rubber polymer; a reinforcing amount of a finely divided, reinforcing, siliceous pigment; and from about 0.1 to 15 parts of an organic coupling agent, said coupling agent per 100 parts of rubber polymer having the general formula A-B, wherein A represents a functional group capable of reacting to bond to the siliceous pigment, and B represents a functional group capable of reacting to bond to the siliceous pigment, and B represents a functional group capable of reacting to bond to the rubber polymer, said functional group B being an active olefinic linkage having an internal double bond.
 11. The tire tread as defined in claim 10 wherein said unsaturated rubber polymer is a member selected from the group consisting of styrene-butadiene rubber, butadiene rubber, and natural rubber.
 12. A tire tread comprising a vulcanized rubber compound including the reaction product of an unsaturated rubber polymer; a finely divided, reinforcing, siliceous pigment; and an organic coupling agent having the general formula A-B, wherein A repResents a functional group capable of reacting to bond to the siliceous pigment, and B represents a functional group capable of reacting to bond to the rubber polymer, said functional group B being an active olefinic linkage having an internal double bond, wherein said rubber compound includes 40 to 90 parts siliceous pigments, 0.3 to 3 parts coupling agent, and 100 parts rubber polymer.
 13. The tire tread as defined in claim 12 wherein said coupling agent is a member selected from the group consisting of 5-triethoxysilyl norbornene, isoprene-triethoxysilane adduct, 4-(triethoxysilyl ethyl)cyclohexene, 6-triethoxysilyl-2-ethylidene norbornane, and 5-triethoxysilyl-2-ethylidene norbornane.
 14. The tire tread as defined in claim 11 wherein the rubber polymer is EPDM and the connecting agent is 5-triethoxysilylnorbornene. 